A known diode includes an n-doped base layer having a cathode side and an anode side opposite the cathode side. On the anode side, a p-doped anode layer is arranged, and a metal layer which functions as an anode electrode is arranged on top of the p-doped anode layer. A higher (n+) doped cathode buffer layer is arranged on the cathode side. A metal layer in the form of a cathode electrode is arranged on top of the (n+) doped cathode buffer layer.
FIG. 1 shows the doping profile of a known p-doped anode layer 5, which includes two sublayers 56, 57. The sublayer 56 is a boron or gallium diffused layer, which has a high maximum doping concentration 565 of around 1*1018/cm3 or higher. Another sublayer with a higher sublayer depth 570 than the other sublayer and a lower maximum doping concentration is an aluminum diffused layer. Due to the high maximum doping concentration 565, the doping profile declines steeply to the sublayer depth 570.
Under fast reverse recovery with steep changes of current (high di/dt), the safe operation area (SOA) of fast recovery diodes is seriously limited by dynamic avalanche breakdown. This is the avalanche breakdown when the electric field is strongly increased by free carriers passing through the high electric field region with saturation velocity. The adjective “dynamic” reflects the fact that this occurs during transient device operation (see S. Linder, Power Semiconductors, EPFL Press, 2006). With increasing recovery di/dt, the dynamic avalanche gets stronger and leads to a device failure under much lower supply voltages as compared to a static breakdown voltage.
Methods for the suppression of dynamic avalanche are based on a proper shaping of the ON-state plasma distribution in the n-base layer of diodes by means of lifetime control. This can be done by single defect peak proton or helium irradiation combined with electron irradiation or heavy metal diffusion, multiple defect peak proton or helium irradiation, or a combination of proton or helium irradiation. Also, a controlled and low anode injection efficiency combined with lifetime control is a possible way to suppress the dynamic avalanche.
The above-mentioned methods are widely used in practice. However, they just remove the effect by decreasing the amount of free carriers passing through the high electric field region and not the cause, which is the high electric field. A method that suppresses the origin of dynamic avalanche and postpones its appearance towards higher supply voltages is based on the introduction of a thick buried low doped p-type layer, which is created by high energy Palladium ion irradiation followed by a diffusion step, and which layer is connected to an anode p-layer (see Vobecky et al, Radiation-Enhanced Diffusion of Palladium for a Local Lifetime Control in Power Devices, IEEE Transactions on Electron Devices, Vol. 54, 1521-1526). The p-layer has very low concentration of acceptors that smoothes out the peak electric field at the anode junction that is responsible for impact ionization leading to an avalanche breakdown. The beneficial effect of this layer increases with increasing thickness while the concentration is kept as close as possible to that of the n-base layer doping. However, this method requires high energy ion irradiation, for which special apparatus are needed. Furthermore, the concentration of the buried P-type layer depends on the quality of the anode surface. For wafers with a large diameter, this method requires delicate application, because a homogeneous temperature distribution during annealing, which is required for a controlled p-layer, is difficult to achieve. There is also a contamination risk during the manufacturing process due to the usage of Palladium.